This invention relates to computed tomography (CT) imaging.
CT imaging is now a standard procedure in medical diagnosis and non-medical applications are being reported with increasing frequency (1), see, for example, Reference No. (1) of the following list; of references:
(1) P. Reimers, W. B. Gilboy and J. Goebbels, "Recent Developments in the Industrial Application of Computerized Tomography with Ionizing Radiation", NDT International, 17 (1984) 197. PA1 (2) L. M. Zatz, "Basic Principles of Computed Tomography Scanning", Radiology of the Skull and Brain, Vol. V: Technical Aspects of Computed Tomography, T. H. Newton and D. G. Potts, Eds. (The C. V. Mosby Company, St. Louis, 1981), pp 3853-3876. PA1 (3) O. Nalcioglu, P. V. Sankar, J. Slansky, "Region-of-Interest X-Ray Tomography (ROIT)", Journal of the Society of Photo-Interpretive Engineers (SPIE), Vol. 206: Recent and Future Developments in Medical Imaging II (1979) pp 98-102. PA1 (4) S. C. Huang, M. E. Phelps and E. J. Hoffman, "Effect of Out-of-Field Objects in Transaxial Reconstruction Tomography", Reconstruction Tomography in Diagnostic Radiology and Nuclear Medicine, M. M. Ter-Pogossian et al., Eds. University Park Press, Baltimore, (1977) pp 185-198. PA1 (5) O. Nalcioglu, Z. H. Cho and R. Y. Low, "Limited Field of View Reconstruction in Computerized Tomography", IEEE Transactions on Nuclear Science, NS-26 (1979) 546. PA1 (6) B. E. Oppeheim, "More Accurate Algorithms for Iterative 3-Dimensional Reconstruction", IEEE Transactions on Nuclear Science, NS-21 (1974) 72. PA1 (7) W. Wagner, "Reconstructions From Restricted Region Scan Data-New Means to Reduce the Patient Dose", IEEE Transactions on Nuclear Science, NS-26 (1979) 2066. PA1 (8) A. Rosenfeld and A. C. Kak, Digital Image Processing, Academic Press, Toronto, 1982. PA1 (9) P. M. Joseph, "Artifacts in Computed Tomography", Ref. 2, Ch. 114, pp 3956-3992. PA1 (10) D. A. Chesler, S. J. Reiderer and N. J. Pelc, "Noise Due to Photon Counting Statistics in Computed X-Ray Tomography", "Journal of Computer Assisted Tomography," 1 (1977) 64. PA1 (11) T. Taylor, N. A. Keller, P. W. Reynolds and S. Shinmoto, "A Computed Tomography System for Studies of Two-Phase Flow", Atomic Energy of Canada Limited proprietary report, CRNL-2976, 1986. PA1 (12) P. D. Toner, G. Tosello, "Computed Tomography Scanning for Location and Sizing of Cavities in Valve Castings", Materials Evaluation, 44 (1986) 203.
CT is a non-invasive technique that can provide an acurate quantitative mapping of the distribution of linear attenuation coefficients inside a body of arbitrary shape and composition.
Referring firstly to FIG. 1 which is a conventional transverse and angular scanning pattern used in CT, the data for a CT image are planar and consist of transverse scans or projections of photon attenuation as a function of position at different angles. After data collection a mathematical process is used to reconstruct a distribution of attenuation coefficients within the object that is consistent with the experimentally measured projections. The reconstruction process most commonly used is termed filtered back projection. See Reference No. (2).
As shown in FIG. 1 the projections span a finite width D. The area that is covered by all projections at different angles is called the field of view. If the centre line of each projection passes through a common point (called the centre of rotation) then the field of view is a circle of diameter D. It is a well known fact that if the field of view does not cover the whole object the parts outside the field of view will cause artifacts in the reconstructed image. See Reference No. (3). This occurs because any part outside the field of view will be intersected by some, but not all, of the projections. If the part outside the field of view is symmetric about the centre of rotation (i.e., an out-of-field annulus) then the projections will be consistent (i.e., all projections will be affected by the annulus in exactly the same way) and the resulting artifact will appear as a circularly symmetric dishing in the field of view. See Reference No. (4). The extent of the dishing will depend on the radius, the thickness, and the attenuation coefficient of the out-of-field annulus. For out-of-field parts other than an annulus, the projections will be inconsistent and the resulting artifact is less predictable.
It is clear from the preceding discussion that in conventional CT even if one wants to examine only a small part of a large object the field of view should cover the whole object. If it were possible, by some means, to restrict the field of view to the region of interest (ROI) without introducing serious artifacts then CT could be used to look at portions of large objects without necessarily increasing the size and number of projections or, alternatively, the resolution inside the ROI could be improved. Restricting the field of view to the ROI also results in a reduced dose to regions outside the ROI. This is of little consequence in non-medical CT but is a major benefit in medical CT. See Reference No. (3). Another advantage of restricting the field of view to the ROI is that matrix size and computation time may be substantially reduced.
A number of investigators have attempted to reduce the effects of out-of-field objects in CT images. See References (3), (5), (6), (7) and U.S. Pat. Nos. 4,189,775; 4,228,505, 4,305,127; 4,333,145; 4,394,738; 4,550,371. Nalcioglu et al. of Reference No. (5) initially used fine sampling in the ROI and coarse sampling elsewhere to produce a coarse reconstruction of the whole object. This was then used to correct the finely sampled portion to obtain a high resolution reconstruction of the ROI only. In a later similar investigation in the same reference they interpolated the coarsely sampled portion and combined this with the finely sampled data before reconstructing the ROI.
Wagner in Reference No. (7) proposed two methods of obtaining images of an ROI from restricted scan data with the objective of reducing patient dose. The first uses scan data of reduced intensity outside the ROI. The second requires no additional data outside the ROI but replaces the missing data by artificial data calculated from the slice outline determined using an optical scanner.